When you say "broken" what do you mean? It won't flush? The button doesn't move? It is running all the time? How is it faulty? The two parts that you have managed to identify are the water inlet valve,and the siphon/flush valve. If the button moves but the toilet does not flush it is going to be the flush valve. However considering that you have to turn off the supply, empty the flush tank and and fit a new flush valve, common sense says change both valves as they are the same age and cost peanuts. If you honestly cannot identify what you need to do the job then you are not up to doing it. It is easy but a fiddly pain, and not something that you want to get wrong. You need the basic tools to even think about it and the ability to recognise the correct screw fittings that are already there to match up with. If you rhink that you can do the job try here. They have all the fittings that you are liable to require and the staff have always been helpful when I need anything. This is the new name for the old Plumb Centre.
https://www.wolseley.co.uk/